1. Field of the Invention
The present invention relates generally to inserts utilized in rock bits and drilling tools and more particularly to the insert material composition and the method of manufacturing the same.
2. Description of the Prior Art
Cemented carbide is widely used as an insert material in TCI rock bits. As used in the following disclosure and claims, the term "cemented carbide" is intended to refer to the type of material resulting when grains of a carbide of the group IVB, VB or VIB metals are pressed and heated in the presence of a binder such as cobalt, nickel or iron as well as various alloys thereof, to produce solid integral pieces. The most common and readily available form of cemented carbide is tungsten carbide containing a cobalt binder. Different carbide grades are utilized in rock bits, the selection of which are dependent on the wear/erosion and mechanical properties thereof. These various properties are described in Assignee's, Grade Properties Handbook, published in 1987. A large portion of the handbook came from World Directory and Handbook of Hardmetals, 2nd Edition, Jetspeed Printing Service Limited, United Kingdom, 1979. These properties are also described in an article entitled Abrasion and Erosion of WC-CO Alloys, found in Metal Powder Report, Vol. 42, No. 12, December 1987. For the most part, the properties of the grade depends on the grain size of the carbide and the binder content. The wear/erosion resistance increases with decreasing carbide particle size and binder content. However, the toughness and impact resistance decreases with decreasing carbide particle size and binder content. As a result, compromises usually have to be made relating to such properties in the selection of materials for inserts.
As also used in the following disclosure and claims, the term "cermet" is intended to refer to a material consisting of ceramic particles bonded with a metal.
A few years ago, it was widely accepted to make inserts from a homogeneous material having uniform grain size. Thereafter inserts were manufactured consisting of a mixture of carbide grain sizes which were cemented using a binder. The wear and erosion resistance of the carbide was changed by the use of bimodal grain size distribution, with an optimum size distribution existing for each binder content.
Manufacturing of the bimodal carbide grades involved mixing and milling the desired amounts of two grain sized carbide particles, preferably tungsten carbide particles, in an attritor or a ball mill with a binder such as cobalt. A liquid media was used in the mill with cemented carbide balls to facilitate good mixing and prevent any oxidation during milling. Wax was generally added in the mill which dissolves in the liquid media. The mills were water cooled. The milling time depended on a number of variables such as the tungsten carbide/cobalt amount, size and the desired mechanical properties. The milled powder was then dried and granulated and sized, which was required for good flowability during pressing. Finally, the granulated powder was pressed and sintered.
It has been found that the various properties mentioned above vary in a linear relationship as the distribution of the two grain sizes vary. For example, the hardness and toughness of a mixture containing a single grain size will steadily vary and change to the hardness and toughness of the other grain size as the amount of the second grain size increases in the mixture. Therefore, in varying the mixture from a pure amount of one grain size, to a pure amount of the second grain size, the hardness and toughness properties will vary in a linear relationship and in an inverse manner.
As a result, although slightly better wear and mechanical properties have been achieved with this process, compromises still had to be made.
Other types of composite carbide inserts have been utilized which have the flexibility of producing products with improved toughness for a given wear resistance and vice-versa.
A number of different approaches to producing these inserts has been taken, but basically, such inserts comprise a coating or layer of hard material bonded to a base member having good toughness qualities. Such contructions are shown in U.S. Pat. Nos. 4,359,335; 4,705,124; and 4,772,405. Some of these constructions have been successful but problems do exist with brazing or bonding such materials together. U.S. Pat. No. 4,604,106 teaches the use of a transition layer between the layers to aid in the bonding.
Another type of construction found in cutting tools utilizes gradient composite metallic structures across the geometry of the cutting structure, such as described in U.S. Pat. No. 4,368,788. However, such a process is limited in application, difficult to control, and quite complex.